1. Field of Invention
The present invention is directed to a method and apparatus for blending process materials and, particularly, to a method and apparatus for blending ultra-high purity chemicals, abrasive slurries and the like.
2. Description of the Related Art
Blended process materials are required, for example, in the pharmaceutical, cosmetic and semiconductor industries. In the semiconductor industry, blended process materials are typically prepared using batch production systems including a dispensing subsystem and a mixing subsystem. The dispensing subsystem transfers materials from a supply source to the mixing subsystem. Supply sources are typically containers designed for safe storage of a process material, such as a chemical or slurry. Other supply sources include facility generation plants, such as deionized (DI) water generation facilities or facilities for supplying other mass-consumed process materials, such as hydrogen peroxide or ammonium hydroxide. The facility generation plant may be directly connected to the dispensing subsystem. A single dispensing subsystem may connect to a variety of supply sources, transferring process material from each to the mixing subsystem.
In the mixing subsystem, the process materials transferred by the dispensing subsystem are added to a mixing vessel or tank. Typically, the materials are added in a predetermined sequence, as required for a particular process. For example, the process may require a predetermined ratio of individual process materials to create a desired blend. The sequence of the process may be based on reactivity or safety, for example an acidic or basic solution is typically added after water. Alternatively, the sequence of the process may be based on the need to reduce or correct process variances, such as dilution of an excessively concentrated wafer polish slurry with DI water. In some cases, it may be necessary to interrupt the addition of a first process material to add a second process material, followed by resumption of addition of the first process material.
The addition of process materials to the mixing vessel is typically monitored and regulated by measuring mass or volume differences. Typical mass difference regulated additions may involve the use of a scale on a holding vessel or tank. In this type of system, each process material is added individually, as an automated control system measuring the mass of the holding vessel is not able to discern the relative amounts of two process materials added simultaneously. Typical volume difference regulated additions may involve the use of flow meters.
Once in the mixing vessel, the process materials are typically blended into a homogeneous solution with an impeller, forming a batch of blended process materials. The batch of blended process materials is typically then used for its intended application.
Many conventional processes require precise addition of process materials to produce a batch of blended process materials that is acceptable for its intended application. Accordingly, the measuring instruments that monitor the inputs to the mixing vessel are typically very precise to insure batch-to-batch consistency. In many applications, even minor process variations may lead to significant differences in the batch of blended process materials, potentially rendering it useless for its intended application.